Are Ionic Bonds Polar or Nonpolar?

The chemical bond is a force of attraction that keeps atoms or ions together. Atoms or ions form new chemical bonds in order to gain stability.

An ionic bond (also known as an electrovalent bond) is formed by the complete transference of electrons from one atom to another.

The atoms involved gain opposite charges and get strongly attracted to each other.

So, what do you think, is an ionic bond polar or non-polar? Come along and find the detailed answer to this often-asked question in this article.

Are ionic bonds polar?

Ionic bonds are typically polar in nature.

An ionic bond is formed by the complete transference of electrons from an electropositive metal atom to an electronegative non-metal atom.

The metal atom converts into a positively charged ion (cation), while the non-metal atom transforms into a negatively charged ion (anion).

The oppositely charged ions are held together by strong electrostatic forces of attraction in an ionic compound.

As a significant electronegativity difference (> 1.6 units) exists between the two reacting species, thus an ionic bond is essentially polar in nature.

The charged electron cloud is largely attracted toward the negatively charged species, leading to a non-uniform charge distribution, hence inducing bond polarity. The ionic bonds possess strong dipole moment values (µ > 0).

Some examples of polar ionic bonds include those formed between Na⁺ and Cl^– (in sodium chloride), Mg²⁺ and 2 Cl^– (in magnesium chloride), Al³⁺ and 3 F^– (in aluminum fluoride), etc.

First, let’s discuss in detail the formation of these ionic bonds so that we can have a better look at their polarity. 

Examples of polar ionic bonds

Sodium chloride (Na⁺Cl^–)

Sodium (Na) belongs to Group I A (or 1) of the Periodic Table. Its electronic configuration is 1s² 2s² 2p⁶ 3s¹. It is an electropositive metal atom with 1 valence electron that it can readily lose so that it gains a stable octet configuration.

The Na-atom loses 1 electron to convert into a Na⁺ cation possessing a complete octet (1s² 2s² 2p⁶).

In contrast, chlorine (Cl) is a halogen present in Group VII A (or 17). Its electronic configuration is 1s² 2s² 2p⁶ 3s² 3p⁵. It is an electronegative (electron-attracting) atom.

So, the Cl-atom obtains a stable octet electronic configuration by gaining 1 more electron in its valence shell (1s² 2s² 2p⁶ 3s² 3p⁶). The Cl-atom converts to Cl^– (chloride) anion.

Na⁺ and Cl^– ions are held together by a strong electrostatic force of attraction known as the ionic bond.

Magnesium chloride (Mg²⁺. 2Cl^–)

The electronic configuration of magnesium (Mg) is 1s² 2s² 2p⁶ 3s². In order to gain a stable octet configuration, the Mg-atom loses 2 valence electrons and converts into Mg²⁺ ion.

These 2 electrons are gained by 2 Cl-atoms, one each that transforms into Cl^– ions.

The oppositely charged Mg²⁺ and 2 Cl^– ions are thus held together by strong ionic bonds.

There are three main factors that largely influence the polarity of the above ionic bonds, namely:

• The electronegativity difference between the bonded atoms or ions
• Dipole moment
• Ionic radius and the magnitude of the charge

Now let’s discuss the effect of the above three factors, one by one on the polarity of the ionic bonds.

Factors affecting the polarity of ionic bonds

Electronegativity

Electronegativity is a measure of an atom’s ability to attract electrons toward itself in a chemical bond. 

In the Periodic Table of elements, electronegativity increases across a period while it decreases down the group.

Therefore, metal atoms (Groups 1-3) are generally less polar than non-metal atoms (Groups 6-7). The less electronegative metal atoms are referred to as more electropositive, i.e., they can easily lose electrons from their valence shell.

The electronegativity of elements is measured on Pauling’s scale, introduced by Linus Pauling.

As per Pauling’s electronegativity scale, a polar covalent bond is formed between two dissimilar atoms having an electronegativity difference ranging from 0.4 to 1.6 units.

 If the electronegativity difference exceeds 1.6 units, the nature of bonding changes from covalent to ionic, where the mutually shared electrons are completely pulled to one side, by the more electronegative specie. 

For instance, a high electronegativity difference of 2.23 units exists between sodium (E.N = 0.93) and chlorine (E.N = 3.16). Hence, the ionic bond between Na and Cl is strongly polar.

Chlorine being more electronegative than sodium, attracts the Na-Cl electron cloud toward itself to a large extent. The Na-atom thus gains a permanent positive charge (Na⁺) while a permanent negative charge (Cl^–) appears on the Cl-atom.

Similarly, a high electronegativity difference of 1.85 units exists between the magnesium (E.N = 1.31) and chlorine (E.N = 3.16) atoms. So, the ionic bond formed between magnesium and chloride ions is also strongly polar in nature.

Dipole moment

Dipole moment (μ) is a vector quantity that points from the positive pole to the negative pole of a bond or a molecule.

 It is mathematically calculated as a product of the magnitude of charge (Q) and charges separation (r). The dipole moment is expressed in a unit called Debye (D).

The dipole moment of a polar covalent bond conventionally points from the positive center to the center of the negative charge.

Hence, the undeniably strong dipole moment of a Na⁺Cl^– ionic bond points from Na⁺ to Cl^– while that of the Mg²⁺ .2 Cl^– bonds are directed from Mg²⁺ to Cl^–.

All ionic bonds are always polar in nature, owing to a significant difference in charge distribution between the two species involved. However, not all ionic bonds are equally polar.

This is because the degree of polarity of an ionic bond depends on the size of the charge and ionic radius.

Ionic radius and magnitude of the charge

The Na⁺Cl^– ionic bond is more polar than Mg²⁺. 2 Cl^–, let’s discuss why.

Atomic radius decreases across the Period. So the radius of magnesium is smaller than that of the sodium atom. A cation always possesses a smaller radius than its parent atom.

The greater the charge on a cation, the smaller its radius. Thus, Mg²⁺ possesses a higher charge and a smaller radius as compared to Na⁺.

A higher charge and a smaller radius result in a higher polarizing effect on the anion.

Mg²⁺ distorts the charged electron cloud of corresponding Cl^– ions. Hence the overall polarity of the ionic bond reduces.

However, the polarizing effect of the cation is reduced in the case of a smaller anion, such as the fluoride (F^–) anion in magnesium fluoride MgF₂. This implies that the ionic bond formed between magnesium and fluoride ions is relatively more polar than that formed between magnesium and chloride ions.

In this way, multiple cation-anion combinations yield ionic bonds polar to a different extent.   

FAQ

Summary

• Ionic bonds are typically polar in nature.
• A high electronegativity difference (> 1.6 units) between two dissimilar atoms leads to the formation of a polar ionic bond.
• The complete transference of electrons takes place from the less electronegative to the more electronegative atom.
• Oppositely charged ions (cations and anions) are formed. Strong electrostatic forces of attraction develop between the oppositely charged ions, called an ionic bond.
• The charged electron cloud stays non-uniformly spread over the two ions, leading to a permanent dipole moment value. Thus, ionic bonds are always polar.
• The degree of polarity of an ionic bond differs based on cationic or anionic radius and size of the charge.

References

1. Diffen. ‘Covalent Bonds vs. Ionic Bonds’’. https://www.diffen.com/difference/Covalent_Bonds_vs_Ionic_Bonds
2. Khan Academy. ‘Ionic bonds’’. https://www.khanacademy.org/science/ap-biology/chemistry-of-life/introduction-to-biological-macromolecules/v/ionic-bonds
3. Study.com. ‘Ionic Character Trend and Bond Polarity’’ https://study.com/learn/lesson/ionic-character-trend-bond-polarity.html
4. ThoughtCo. ‘Polar Bond Definition and Examples’’. www.thoughtco.com/definition-of-polar-bond-and-examples-605530